return g;
        }

        List<MetadataGraphVertex> visited = new ArrayList<>(g.getVertices().size());
        visit(g.getEntry(), visited, g);

        List<MetadataGraphVertex> dropList = new ArrayList<>(g.getVertices().size());

        // collect drop list
        for (MetadataGraphVertex v : g.getVertices()) {
            if (!visited.contains(v)) {
                dropList.add(v);
            }
        }

     */
    protected void extractFrontMatterMetadata(final Node document, final ExtractData extractData) {
        final YamlFrontMatterVisitor visitor = new YamlFrontMatterVisitor();
        document.accept(visitor);

        final Map<String, List<String>> frontMatter = visitor.getData();
        for (final Map.Entry<String, List<String>> entry : frontMatter.entrySet()) {
            final String key = "frontmatter." + entry.getKey();

        ClasspathContainer cpc;

        List<MetadataGraphVertex> visited;

        // -----------------------------------------------------------------------
        protected ClasspathGraphVisitor(MetadataGraph cleanGraph, ClasspathContainer cpc) {
            this.cpc = cpc;
            this.graph = cleanGraph;

            visited = new ArrayList<>(cleanGraph.getVertices().size());
        }

        return queryBuilder.rewrite(queryShardContext);
    }

    @Override
    public void visit(final QueryBuilderVisitor visitor) {
        queryBuilder.visit(visitor);
    }

    @Override
    public XContentBuilder toXContent(final XContentBuilder builder, final Params params) throws IOException {

        accessResult.setUrl("https://example.com/visited");
        accessResultMap.put("https://example.com/visited", accessResult);

        UrlQueueImpl<Long> newUrl = new UrlQueueImpl<>();
        newUrl.setSessionId(sessionId);
        newUrl.setUrl("https://example.com/new");

        UrlQueueImpl<Long> visited = new UrlQueueImpl<>();
        visited.setSessionId(sessionId);
        visited.setUrl("https://example.com/visited");

     *
     * @param sessionId The session ID.
     */
    void saveSession(String sessionId);

    /**
     * Checks if a URL has been visited.
     *
     * @param urlQueue The URL queue.
     * @return true if the URL has been visited, otherwise false.
     */
    boolean visited(QUEUE urlQueue);

    /**
     * Generates URL queues.
     *
     * @param previousSessionId The previous session ID.

    limiter.acquire(Integer.MAX_VALUE / 4);
    limiter.acquire(Integer.MAX_VALUE / 2);
    limiter.acquire(Integer.MAX_VALUE);
    assertEvents("R0.00", "R0.00", "R0.00");

    limiter.setRate(1.0);
    limiter.acquire();
    limiter.acquire();
    limiter.acquire();
    assertEvents("R0.00", "R1.00", "R1.00");

    limiter.acquire(Integer.MAX_VALUE / 4);
    limiter.acquire(Integer.MAX_VALUE / 2);
    limiter.acquire(Integer.MAX_VALUE);
    assertEvents("R0.00", "R0.00", "R0.00");

    limiter.setRate(1.0);
    limiter.acquire();
    limiter.acquire();
    limiter.acquire();
    assertEvents("R0.00", "R1.00", "R1.00");

    public void saveSession(final String sessionId) {
        // NOP
    }

    /*
     * (non-Javadoc)
     *
     * @see org.codelibs.fess.crawler.service.UrlQueueService#visited(UrlQueue)
     */
    @Override
    public boolean visited(final UrlQueueImpl<Long> urlQueue) {
        final Queue<UrlQueueImpl<Long>> urlQueueList = dataHelper.getUrlQueueList(urlQueue.getSessionId());
        synchronized (urlQueueList) {

        final CrawlerContext crawlerContext = CrawlingParameterUtil.getCrawlerContext();
        final UrlQueueService<UrlQueue<?>> urlQueueService = CrawlingParameterUtil.getUrlQueueService();
        if (!urlQueueService.visited(urlQueue)) {
            if (checkAccessCount(crawlerContext)) {
                final AccessResult<?> accessResult = createAccessResult(responseData, resultData);
                if (logger.isDebugEnabled()) {